This invention relates to acousto-optic modulated gas lasers.
When a laser beam is to be modulated at a rate between a few kilohertz and one megahertz, the modulator can be positioned within the laser cavity. However, in order to modulate at rates in excess of one megahertz, the standard modulation method employs an extra-cavity acousto-optic or electro-optic modulator. Because of cost considerations, the acousto-optic modulator is preferred over the electro-optic type.
An acousto-optic modulator is composed of a propagation medium which exhibits a change in the index of refraction due to mechanical strain. A piezoelectric transducer may be attached to the modulator to generate ultrasonic acoustic waves. The ultrasonic waves propagating through the modultor medium produce mechanical strain which changes the refractive index of the medium. These mechanically induced changes in the refractive index cause light passing through the medium to be diffracted. The intensity and frequency of the diffracted light are determined by the strain which, in turn, is determined by the modulation signal applied to the transducer. The rise time of acousto-optic modulators is determined by the transit time of the acoustic wave through the laser beam. The rise time may be improved by decreasing the diameter of the laser beam.
Heretofore, the laser beam was conventionally emitted from the end of the laser tube having a spherical mirror. The acousto-optic modulator was spaced from that end of the laser tube. The rise time of the modulator was enhanced by employing focusing means between the laser tube and the modulator. Thus, as the laser beam passed through the modulator, it was focused down to a relatively small diameter. For maximum modulation efficiency, the modulator was positioned so that the acoustic wave intersected the laser beam at the Bragg angle. The Bragg angle .theta. being defined as: ##EQU1## WHERE .lambda. IS THE LASER WAVELENGTH AND .lambda. IS THE WAVELENGTH OF THE ACOUSTIC WAVE. This resulted in quite a complexly aligned system for directing and shaping the laser beam. All of the separate elements of the system have to be maintained in the proper relationship to one another.